Preheat the oven to 190°C/gas mark 5, with two flat preheated baking trays inside. In a heavy-based pan, fry the streaky pancetta in a little olive oil until crispy, then remove on to kitchen towel to drain the excess fat.
In the same pan, cook the leeks and thyme in the pancetta fat for 5-7 minutes, until soft but still green in colour. Add the pancetta back in along with the mushrooms, shredded chicken and plain flour and stir until fully combined. Turn up the heat, add 800ml boiling water, season with salt and pepper and allow to bubble away for a couple of minutes.
Remove from the heat, add the yogurt and stir through. Place a sieve over a bowl and pop the mixture in the sieve to drain the excess liquid (you can use this as gravy later!). Pop the drained mixture into individual pie pots and allow to cool fully.
To make the gravestones, you'll need the two, preheated flat baking trays. Using the end of the puff sheet, cut out two gravestone shapes and place in-between two pieces of greaseproof paper, sandwiched in-between the two trays – this makes sure they stay flat. Cook for 15-20 minutes until golden and remove to cool.
Now the mix is completely cool, brush the rim of the pie pots with the egg wash and cut out two circles of puff pastry roughly the same size as the pots. Cut a slit approximately the same width as the grave stones 3/4 of the way up the circle, then press them on to the pot. Press down with a fork and lightly score the pastry in a crisscross. Egg wash the pastry and pop in the oven for 25-30 minutes until golden.